Three antimycin resistant mutants of Saccharomyces cerevisiae are characterized genetically. The mutations have been shown to be cytoplasmically inherited by four criteria. The phenotype persists in diploids formed by a cross with a pO strain of yeast of the opposite mating type. Diploids heterozygous for the antimycin marker, however, show segregation of the resistance and sensitivity during mitosis. Tetrad analysis indicates a non-Mendelian segregation (4:0 and 0:4) of the mutations. The antimycin marker can be eliminated by ethidium bromide treatment under conditions that should have deleted all of the mitochondrial DNA.